Brain Mechanisms of Thirst

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The concept of "homeostasis" (the idea that the body tends to balance various factors to maintain a stable internal state) is a key part of most current explanations of ingestive behavior, in particular water and salt intake. However, a number of observations, including Dr. Rowland's own, suggest that this is too simple a view of the mechanism and organization of ingestive behaviors. This research project will critically examine the homeostatic model in several new ways, including the use of a sham-drinking rat preparation in which ingested fluids are lost through a tube from the stomach out of the animal. Dr. Rowland and others have shown that water deprivation increases sham drinking more than normal drinking. Surprisingly, however, Dr. Rowland has been unable to produce this greater increase in sham drinking with the presumed elementary thirst stimuli, hypovolemia (low blood volume) and hyperosmolarity (high concentration of chemicals dissolved in the blood). The first experiment will use different routes of administration of salt at different times of day in order to assess the generality of the surprising finding. Dr. Rowland will also examine the microstructure (bursts and pauses) of drinking and attempt to assess the role of very-short-term feedback loops on oral satiety that may override physiologic thirst signals (and thus be independent of the controls of fluid homeostasis). While such oral satiety factors have been suspected for many years, they have never been analyzed convincingly. The second and third experiments will deal with oral versus systemic factors in the control of salt intake, again using sham ingestion as an experimental tool to separate oral from systemic (pre- and post-ingestional) factors. At least two different strains of rats will be studied because of genetic differences in salt preference. Dr. Rowland hopes to be able to assess, from microstructural analysis of sham drinking, the mechanisms by which this genetic difference affects behavior. Dr. Rowland's studies place a new theoretical importance on oral factors as primary mechanisms in maintaining fluid balance and have wider implications for the interplay between systemic states and environmental stimuli in the control of behavior.